Recent years have witnessed practical use of a flat-panel display in various products and fields. This has led to a demand for a flat-panel display that is larger in size, achieves higher image quality, and consumes less power.
Under such circumstances, great attention has been drawn to an organic EL display device that (i) includes an organic electroluminescence (hereinafter abbreviated to “EL”) element which uses EL of an organic material and that (ii) is an all-solid-state flat-panel display which is excellent in, for example, low-voltage driving, high-speed response, and self-emitting.
An organic EL display device includes, for example, (i) a substrate made up of members such as a glass substrate and TFTs (thin film transistors) provided to the glass substrate and (ii) organic EL elements provided on the substrate and connected to the TFTs.
An organic EL element is a light-emitting element capable of high-luminance light emission based on low-voltage direct-current driving, and includes in its structure a first electrode, an organic EL layer, and a second electrode stacked on top of one another in that order, the first electrode being connected to a TFT. The organic EL layer between the first electrode and the second electrode is an organic layer including a stack of layers such as a hole injection layer, a hole transfer layer, an electron blocking layer, a luminescent layer, a hole blocking layer, an electron transfer layer, and an electron injection layer.
A full-color organic EL display device typically includes organic EL elements of red (R), green (G), and blue (B) as sub-pixels aligned on a substrate. The full-color organic EL display device carries out an image display by, with use of TFTs, selectively causing the organic EL elements to each emit light with a desired luminance.
Such an organic EL display device is produced through a process that forms, for each organic EL element serving as a light-emitting element, a pattern of a luminescent layer made of an organic luminescent material which emits light of at least the above three colors.
Such formation of a luminescent layer pattern is performed by a method such as a vacuum vapor deposition method that uses a vapor deposition mask referred to as a shadow mask.
A mask is typically equivalent in size to a film formation substrate. When in use, the mask is fixed to a mask frame under tension for prevention of bending and distortion and is closely attached to the film formation substrate.
However, in a case where an organic EL display device is produced by such a vacuum vapor deposition method, a larger substrate size of a film formation substrate normally allows a larger number of panels to be formed from a single film formation substrate, and thus reduces the unit cost of a panel. This means that a larger sized film formation substrate allows an organic EL display device to be produced at a lower cost.
Unfortunately, a larger substrate size requires the mask to be larger in size as well. Such a larger size results in a gap opening between the film formation substrate and the mask due to self-weight bending and elongation of the mask. This makes it impossible to form a pattern with high positional accuracy, and thus causes misplacement in vapor deposition and color mixture, thereby making it difficult to achieve high resolution.
Further, a larger substrate size requires the mask and a mask frame that holds it to be both extremely large and heavy. This in turn requires a device that uses the mask to be extremely large and complex, which not only makes it difficult to design such a device, but also causes a safety problem in handling the mask during a production step or a step such as replacing the mask.
On the other hand, there has been known a method that performs vapor deposition by use of a mask which is smaller in size than a film formation substrate, so that a vapor-deposited film is formed on a large-sized film formation substrate that is used as the film formation substrate.
For example, Patent Literature 1 discloses a method that uses a mask which is smaller in size than a film formation substrate, the method performing vapor deposition over an entire surface of the film formation substrate while shifting the mask and a vapor deposition source relative to the film formation substrate.
Thus, the use of a vapor deposition mask which is smaller in size than a film formation substrate solves the problem caused by the use of a mask equivalent in size to a film formation substrate.
Citation List
[Patent Literatures]
Patent Literature 1
Japanese Patent Application Publication, Tokukai, No. 2004-349101 A (Publication Date: Dec. 9, 2004)
Patent Literature 2
Japanese Patent Application Publication, Tokukai, No. 2002-235166 A (Publication Date: Aug. 23, 2002)
Patent Literature 3
Japanese Patent Application Publication, Tokukai, No. 2003-133067 A (Publication Date: May 9, 2003)